(CNN) -- A suspicious piece of luggage that was about to be loaded onto a plane at Windhoek Hosea Kutako International Airport in Namibia contained no explosives, a spokeswoman for the airline said Thursday.

"Air Berlin has confirmed that there was no explosive material in the laptop bag found in Namibia," Sabina Teller, spokeswoman for Air Berlin, told CNN. "The luggage was found in the airport, at no point was on the plane. It had no luggage label so it was impossible to know where it was going, which company it was supposed to fly with or who it belonged to."

The laptop bag, found Wednesday morning among the luggage being loaded onto flight 7377 to Munich, Germany, contained a functioning electronic clock with wiring attached, suitable for use in a bomb, sources close to the investigation told CNN Thursday. The sources asked not to be identified because of the sensitivity of the investigation.

The bag was discovered at a luggage screening point by security officials, who then notified the Namibian police explosives unit.

After it was verified that the bag contained no explosives, passengers were asked to get off the plane to identify their luggage, but no one claimed ownership of the laptop bag, the source said. The flight resumed about six hours later to Munich, but some cargo was held back for investigation by officials from Namibia, Germany and the United States.

The Namibia Airports Company, which runs the Windhoek airport, said the item was found at the luggage screening point prior to loading, though it did not specify whether the item was supposed to be loaded onto the Air Berlin flight.

The 296 passengers and all the bags on the flight were subjected to additional security checks before the flight took off. It landed safely in Munich after a six- or seven-hour delay, said Air Berlin spokeswoman Silke Manitz.

Germany raised its threat level on Wednesday, saying concrete evidence had emerged of a possible attack planned there later this month and of persistent efforts by Islamists to attack it. A senior German counterterrorism source told CNN on Thursday that the German Interior Ministry warning was linked to recent threats to the country from al Qaeda in Pakistan. Interior Minister Thomas de Maiziere said there was "reason for concern but no reason for hysteria."

"Security authorities are considering three different threat scenarios by international terrorists," he said. "Security authorities have been pursuing these indications for weeks with the highest intensity and proper sensitivity -- in closest cooperation with our international partners.

"Now further insights and facts relevant to danger have added themselves to these established insights," he said Wednesday. The "new situation" prompted authorities to raise the threat level.

Last month, the United States issued a travel alert for Americans in Europe amid concerns that al Qaeda or related groups might be planning attacks similar to the 2008 massacre in the Indian city of Mumbai.

CNN's Laura Perez Maestro in London; Ben Brumfield in Atlanta, Georgia; Diana Magnay in Hamburg, Germany; John Grobler in Namibia; and Nkepile Mabuse in Johannesburg, South Africa, contributed to this report.